The present invention relates, as stated in the title, to improved seal for piston screws in plastics injection and extrusion apparatus, the new constructional, shape and design features of which fulfill the mission for which it has been specifically devised with maximum reliability and efficiency.
In the plastics industry, and more particularly in the sector using plastics injection machines, the problem existing in the screw end sealing systems (movable valves, seals with ball valves and others) during the injection cycle, when the end of the screw is acting as a piston, is well-known. Such problems are always derived from a premature wear of the friction areas of the facing surfaces, that is the rear portion of the tip and the front portion of the concentrically mounted valve, when the plastics material is charged and is transferred to the front injection chamber.
The abovementioned valve is mounted in ring fashion around the tip stem and slides backwards when the piston pushes the screw to cut off the supply of plastic material to said front injection chamber.
The movement of the valve over the tip or on the front portion of the screw is the cause of the said frictional wear due of the high tonnage pressure transmitted by the piston to the screw and by the latter to the tip and the valve, all of which means that both the tip and the valve must be frequently replaced, as a result of the play that the friction between the facing surfaces finally produces.
The replacement of the tip and valve causes periodic stoppages during which it is necessary to dismantle the injection barrel and remove the screw, together with the tip and the valve. All of this causes downtime, together with the corresponding costs of assembly and dismantling and replacement of the said tip and valve. Thus production is completely interrupted during said downtime and the injection machine becomes unproductive until the worn tip and valve have been replaced.